1. Field of the Invention
This invention relates to ringing signal generators for telephone systems and, more particularly, to the remote generation of ringing signals for subscriber loop pair gain systems.
2. Description of the Prior Art
It has become increasingly common to utilize carrier techniques to establish communication paths between a telephone central office and a plurality of remotely located telephone subscribers. Where such systems were at one time economically justifiable only for extremely long rural routes, today, due to the rapidly decreasing costs of integrated microelectronics, such systems are becoming economically feasible for shorter and shorter subscriber loops.
In a subscriber loop carrier system, a plurality of voice channels are derived on one or two pairs of wire conductors by using analog carrier frequency division techniques or by using time division digital techniques. In either case, the metallic connection previously used for transmitting subscriber ringing signals to the subscriber location can no longer be used for this purpose. It has therefore become necessary to generate ringing signals at the remote terminal of the carrier system and to control the generation and application of these ringing signals by supervisory information transmitted over the carrier-derived channels.
Since remote ringing signal generators serve a relatively small number of subscribers, must be housed at the remote location and are preferably powered from the central office, it is desirable that such ringing signal generators be small, inexpensive, compact and require little power. One such system is shown in the copending application of B. S. Bosik, Ser. No. 974,384, filed Dec. 29, 1978.
One convenient way to generate ringing signals at the remote location is that shown in the above-mentioned Bosik application. A direct current voltage of high magnitude is generated from the central office battery voltage using dc-to-dc conversion techniques. Semiconductor switches can then be used to interrupt this direct current voltage at a ringing signal rate. The resulting pulses of voltage are applied to the subscriber drop wires to operate the mechanical ringer in the subscriber's telephone set. Unfortunately, the high voltage square waves generated in this way include many high frequency harmonics which induce audible frequency voltages in adjacent wire pairs. These induced voltages (crosstalk) interfere with the use of such adjacent pairs by other telephone subscribers.